An independent Scotland would face “incredible expense” policing its borders against terrorists, organised criminals and illegal immigrants, Unionists warned today as a new analysis detailed the extent of the current safeguards.

Alistair Carmichael, the Scottish Secretary, will this afternoon publish a report demonstrating how more than £2 billion a year is spent and 20,000 staff keeping the country safe from external threats.

The document also provided the number of cross-Border travellers whose journeys could be disrupted if Alex Salmond pressed ahead with his plan to lift restrictions on non-EU immigration following independence.

UK ministers have previously warned this would be forced to erect border posts to prevent immigrants using Scotland as an easy “back door” into England.

According to today’s analysis, around 23 million vehicles per year cross the Scotland-England border unhindered, along with seven million rail passengers and 32.4 million tons of freight.

The First Minister has argued that Scotland needs higher levels of immigration to stimulate economic growth but the report said the current visa system allows skilled foreigners to enter this country in a carefully controlled manner.

Speaking ahead of the report’s publication, Mr Carmichael said: “The latest Scotland Analysis provides a long list of benefits all of the UK gets from managing our border together. It is a huge operation which helps trade, stops crime and supports tourism.

“All of this would be put at risk by those obsessed with independence who can't explain how their immigration and borders plans would work.”

Alex Johnstone, a Scottish Tory MSP, said the civil service-produced document highlighted the “tremendous work” carried out by the UK Border Agency on behalf of Scotland.

“But it also shows the incredible expense a separate Scotland would have to go to in order to replicate it to the same standard,” he said.

“An understanding on travel arrangements across the border would be desirable, but if the Scottish Government comes up with a more liberal approach on immigration, controls and patrols would have to be established.”

The Whitehall analysis said that in 2011/12 the UK Government processed 21,875 asylum claims, patrolled more than patrolled over 63,000 miles of territorial waters and searched more than 2,000 vessels.

Officials also prevented more than four-and-a-half tons of drugs from entering the country, intercepted 455 million smuggled cigarettes and oversaw international trade worth £573 billion.

Employers can take on foreigners from outside Europe using an “extensive visa system designed to ensure only those with the skills the economy needs are allowed to migrate to the UK.”

The analysis said this took account of Scotland’s “particular circumstances and need for skilled labour.”

The document will also address what would happen to British citizenship and passports if Scots vote for independence. They can currently access consular support if they require assistance in more than 170 countries.

Despite wanting to allow much higher immigration, Mr Salmond has said he wants a separate Scotland to be in a Common Travel Area with the UK and Ireland.

There are currently no passport controls between Northern Ireland and the Republic of Ireland but only because the latter's immigration policies deliberately do not diverge greatly from Westminster's.

Nicola Sturgeon, the Deputy First Minister, said the report lacked “all credibility” and ignored the SNP administration’s plans for a Scottish Border and Migration Service.

“And secondly, it completely ignores the reality of the Common Travel Area, in which the UK and Ireland already have no border controls but differing immigration policies,” she added.

“Indeed, the only people wanting to erect borders are Mr Carmichael's own colleagues in the Westminster Government, who are threatening to drag Scotland and the UK out of the EU and the single market.”